THE CIRCULATION OF THE BLOOD. 327 



quotient by 0.188. The quotient of the first two values represents 

 the distance a body would have to fall to acquire this velocity: 

 viz., 0.0127 meter. The work done is therefore 0.188 X 0.0127, or 

 0.0023 kilogrammeter. 



The entire work of the left ventricle is the sum of these two 

 amounts, or 0.604 kilogrammeter. Assuming that the heart beats 72 

 times per minute, the work done daily would be 0.604 X 72 + 

 60 X 24, or 62.622 kilogrammeters. The right ventricle approxi- 

 mately performs about one-third of this amount of work in over- 

 coming the resistance offered by the pulmonary system and in 

 imparting velocity to its contained volume of blood. The work of 

 the entire heart would therefore be for the twenty-four hours about 

 83.496 kilogrammeters. 



THE NERVE MECHANISM OF THE VASCULAR APPARATUS. 



The middle coat of the arteries, and especially of those in the 

 peripheral "region of the arterial system, consists of a well-defined 

 layer of non-striated muscle-fibers arranged in a circular direction or 

 at right angles to the long axis of the vessel. In the physiologic con- 

 dition these fibers are in a state of continuous contraction, more or 

 less pronounced, and give to the arteries a certain average caliber 

 which permits a definite volume of blood to flow through them in a 

 given unit of time. 



The cause of this tonic contraction is not definitely known. It 

 has been attributed to the action of local nerve-ganglia, to the pres- 

 sure of blood from within, to the influence of organic substances in the 

 blood, the products of gland activity: e. g., adrenalin or epinephrin. 



This tonic contraction of the vascular muscle is subject to increase 

 or decrease in accordance with the action of various agents. In- 

 creased contraction will result in a decrease of the caliber and a 

 reduction in the outflow of blood. Decrease of the contraction or 

 relaxation will result in an increase both of the caliber and outflow 

 of blood. The small arteries thus determine the volume of blood 

 passing to any given area or organ in accordance with its functional 

 activities. 



The Vaso-motor Nerves. The activities of the vascular muscle 

 are regulated by the central nerve system through the intermedia- 

 tion of nerve-fibers, termed vaso-motor nerves. Of these there are 

 two kinds, one which increases or augments the contraction, the 

 vaso-constrictors or vaso-augmentors ; another which decreases or 

 inhibits the contraction, the vaso-dilatators or vase-inhibitors. 



I The vaso-motor nerves of both classes, unlike the ordinary motor / 

 nerves, do not pass directly to the muscle-fiber, but indirectly by way 

 of the ganglia of the sympathetic nervous system. < In these ganglia 



